This invention relates to an apparatus and method for purifying air within a clean room that has been contaminated with fine particles or trace amounts of gases or ions.
The use of "clean rooms" is not limited to high-technology areas including the semiconductor industry, precision machine/engineering industry, photographic industry, and the pharmaceutical manufacturing industry and hospitals (where they are used as "biological clean rooms"). Today, the use of clean rooms has expanded to the food industry, agriculture and even to their peripheral fields. The working environments of these industrial areas should satisfy various conditions, among which temperature, humidity, air stream, as well as air purification are extremely important.
For purifying the air to be used in those industries, high-performance filters are used, such as high-efficiency particulate air (HEPA) filters which are composed of glass fibers, and more efficient ultra-low penetration air (ULPA) filters. Prefilters of these high-performance filters are also used and common types are moderate-performance, coarse dust filters which are composed of synthetic fiber other than glass fibers.
These filters are intended to reject particles in air and are capable of efficient rejection of fine particles as small as about 0.1 .mu.m. However, these filters which depend upon the physical sifting action for the removal of fine particles have difficulty in rejection gases or ions.
In the existing LSI fabrication plants, the contamination of the surfaces of semiconductor wafers is held to be caused by either fine particles or gases and ions. Contamination by gases and ions is particularly important since it causes such serious problems as increasing the contact resistance of semiconductors or affecting the bulk characteristics of semiconductor wafers.
There are various sources of gases and ions; for example, they are generated in semiconductor fabrication steps such as etching or they originate from the finishing materials of the clean room or they are brought in when atmospheric air is introduced into the fabrication process. The gases or ions once introduced into the clean cannot be rejected by the conventional air purifying system and they build up progressively in the room to cause potential adverse effects not only on the quality of the final product but also on the health of the operator.
Hence, rejection of the gases or ions that have entered the clean room is of a particularly great concern today.
As mentioned above, no effective method is currently available to remove the trace amounts or gases of ions that have been introduced into the clean room, except that active charcoal filters as well as magnanese oxide supporting catalyst filters are known as gas or ion removing means.
Charcoal exhibits a certain degree of ability to remove various gases but its ability to absorb polar gases is poor, particularly in the case of basic gases such as ammonia. Further, the mechanism of adsorption of gases on charcoal is by physical adsorption, so if the amount of gas adsorption increases, the chance of re-liberation of the adsorbed gas will also increase. In addition, charcoal will easily disintegrate into a powder form and can potentially become a source of generation of fine particles.
Manganese oxide supporting catalyst filters are too low in the efficiency of gas removal to be suitable for the purpose of removing gas components that are present at extremely low concentrations in the clean room. On the contrary, it has been reported that the filters are sources of generation of harmful gases in trace amounts.
Besides the use of such filters, wet adsorbers which use liquid chemicals are also known as a means of removing gases or ions. Such adsorbers are bulky and require cumbersome procedures of maintenance and management. In addition, they were initially developed as apparatus for removing high concentrations of gaseous components and, hence, are not suitable for the purpose of removing gaseous components that are present in trace amounts.
Japanese Patent Public Disclosure No. 12315/1988 proposed a method for removing acidic or alkaline gases by means of a filter formed from a blend of ion-exchange fibers and glass fibers. This method is applicable to the purification of air in household or business offices but it cannot be applied with great effectiveness to the clean room in the semiconductor industry and other sites where rigorous performance requirements need be satisfied. This is because i) the ion-exchange fibers per se are frequent sources of dust generation, ii) the ability to remove fine particles is unsatisfactory, and iii) high susceptibility to moisture and other factors instabilizes the ability to remove gaseous components.
Japanese Patent Public Disclosure Nos. 60710/1991 and 60711/1991 proposed shaping mixtures of ion-exchange fibers and non-ion-exchange fibers into filters that were primarily intended to remove ion components. The materials to be rejected by these filters are somewhat different from the filter proposed by Japanese Patent Public Disclosure No. 12315/1988, supra but one may well consider that the two inventions have technically the same problems.